1. The Field of the Invention
The invention relates to transmitting bits representing digital information from a mobile terminal to a central baseband unit.
2. The Relevant Technology
Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Cellular telecommunication systems, also known as mobile networks, typically comprise a radio access network (RAN) coupled to a core network (CN) and a plurality of mobile terminals that are communicatively coupled via a radio link to the RAN. In this way mobile terminals may be coupled to core network via the radio access network. A plurality of radio access network types are known, e.g. the GSM radio access network GRAN, the UMTS radio access networks UTRAN and the E-UTRAN of the so-called LTE network.
A radio cell or a sector of a radio cell is the area defined by the radio coverage of an antenna located at a so-called base station (BS), also known as base transceiver station (BTS). Note that in the following description the term base transceiver station (BTS) shall equally describe so-called eNodeBs or any other radio communications station installed at a fixed location providing the radio interface for the coupling to the mobile stations and used as the terminating end of the radio access network of a cellular communications system.
Besides transmitting radio signals downlink, i.e. from the network side to the mobile terminal, a base transceiver station receives radio signals transmitted uplink, i.e. directed from a mobile terminal to the network side of the communication system. The received radio signal may undergo some analog processing, e.g. the received analog signal may be amplified by a low-noise amplifier LNA, before being digitized by an analog-to-digital circuit. Once the received signal has been digitized, i.e. the signal is represented by complex values, it can be further processed using digital circuitry. For example in early conventional base transceiver stations, e.g. in the GSM system, the received signal is demodulated thus retrieving the transmitted binary information that in turn was forwarded to the network for further routing and processing.
For mobile radio networks so-called cloud architectures have come to the focus of attention. In such networks signal processing that is usually executed in remote base stations is executed in a centralized baseband unit (BBU). One example is the so-called Cloud Radio Access Network (C-RAN) where base stations are degraded to mere remote radio heads (RRHs) that in the uplink only down-convert the received signals from the respective carrier frequency to baseband frequency and then digitize the down-converted received signal by sampling and quantizing thus producing a stream of digital complex I/Q samples. Said stream of I/Q samples is forwarded via a fast high capacity wired or wireless link towards the centralized baseband unit BBU where the signal is dequantized to reconstruct the signal that was originally received at the remote radio head. The signal processing performed in the BBU thus is similar to that in a conventional base transceiver station including the demodulation of the reconstructed received signal and the decoding of the access channel's forward error correction (FEC), but wherein the baseband unit receives and processes a plurality of I/Q sample streams from a corresponding plurality of RRHs.
The centralized processing of signals originating from a plurality of remote radio heads may provide for improved energy efficiency, scalability, a potential benefit from load-balancing and joint processing of the plurality of received signals.
However, the quantization of a received signal introduces distortion into the quantized signal, i.e. due to the discrete quantization intervals an error is introduced by the quantization and dequantization process. To mitigate these effects quantizers are designed for low quantization errors.
In addition, the use of quantizers and particularly the use of corresponding dequantizers comes at the cost of losing soft information during quantization. When considering that transmitted digital data is not merely represented by hard bits with a value of either “0” or “1”, but instead is associated with information how reliable the value of a transmitted bit can be decoded. This information is known as soft information and typically reflects the probability of a bit being 1 or 0. In one embodiment this soft information is represented by a logarithmic likelihood ratio, i.e. so-called LLR. The soft information of a transmitted bit can be computed during detection if statistical knowledge of the random process that underlies the transmission, i.e. the signal to noise ration SNR of a considered average white Gaussian noise channel, AWGN, is available.
Furthermore, when transmitting a quantized value via a forwarding channel, the soft information associated with the quantized value, i.e. the quantization bits, typically is not considered at the receiver side of the forwarding channel during dequantization and is thus unavailable for any subsequent processing, thus increasing the end-to-end bit error rate.
Hence there is a need to at least partially improve the processing of uplink signals using centralized processing.